Base station assistance for random access performance improvement

ABSTRACT

Methods and apparatus are described. According to a method, a wireless transmit/receive unit (WTRU) acquires a first random access configuration and a second random access configuration, which are different, and performs a first random access procedure using the first random access configuration using first physical resources and a second random access procedure using the second random access configuration using second physical resources. The first random access configuration is for at least initial random access and the second random access configuration is for timing adjustments. The first physical resources are different than the second physical resources. The WTRU receives a first message for the first random access procedure and a second message for the second random access procedure. The first message is an aggregated response message including a plurality of first random access responses and the second message is an aggregated response message including a plurality of second random access responses.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/090,070, filed on Apr. 4, 2016, which is a continuation of U.S.patent application Ser. No. 14/766,157, filed on Apr. 23, 2010, nowabandoned, which claims the benefit of U.S. Provisional Application No.61/172,072, which was filed on Apr. 23, 2009, and U.S. ProvisionalApplication No. 61/183,700, which was filed on Jun. 3, 2009, thecontents of which are hereby incorporated by reference herein.

FIELD OF INVENTION

This application is related to wireless communications.

BACKGROUND

Two issues that currently exist in wireless broadband systems arelatency of random access (RA) failure detection on the subscriber sideand collisions. An RA attempt fails when the BS fails to receive the RAsignal correctly either due to collision, i.e., two or more users(subscribers) use the same RA opportunity, or due to an insufficientsignal level.

The first issue is the latency issue of RA failure detection on thesubscriber side. In scheduling-based access systems, RA accommodates newaccess needs, e.g., new users, existing users' new requirements, etc.Broadband Wireless Access systems, e.g., worldwide interoperablility formicrowave access (WiMAX), and long term evolution (LTE), are typicalscheduling-based access systems, where a base station (BS) controls theuse of the air link resources.

When a RA failure occurs, the subscriber needs to detect it and thentake actions accordingly, e.g., retry, or ramp up its transmissionpower. A commonly-used mechanism for a subscriber to detect the failureof its random access attempt is timer-based, i.e., after waiting apre-defined time period without getting the expected response, thesubscriber assumes that its previous RA attempt failed, where theexpected response depends on the purpose of the RA.

The pre-defined time period may play a role in a timer-based RA failuredetection mechanism because the recovery from a RA failure has to waitfor the pre-defined time that needs to be sufficiently long enough toprocess the RA request receptions and responses under the worst-caseconsiderations, e.g., the heaviest traffic loading.

The second issue relates to collision scenarios. A collision occurs whentwo or more subscribers choose the same RA opportunity, where a RAopportunity refers to the opportunity for a subscriber to send a RArequest. For example, a RA opportunity in IEEE 802.16 systems consistsof a RA channel and a RA code sent on the RA channel. When a collisionoccurs, there are several possible results. The first result is that theBS detects nothing. The second result is that the BS detects acollision. The third result is that a single RA Request is erroneouslydetected by the BS in the RA opportunity with collision.

SUMMARY

Methods and apparatus are described herein. According to a method, awireless transmit/receive unit (WTRU) acquires a first random accessconfiguration and a second random access configuration. The first randomaccess configuration and the second random access configuration aredifferent and each corresponds to different physical resources. The WTRUalso performs a first random access procedure using the first randomaccess configuration and a second random access procedure using thesecond random access configuration. The first random accessconfiguration is used for at least initial random access and the secondrandom access configuration is not used for initial random access andnot used for handover random access.

BRIEF DESCRIPTION OF THE DRAWINGS

A more detailed understanding may be had from the following description,given by way of example in conjunction with the accompanying drawingswherein:

FIG. 1 shows an example wireless communication system including aplurality of WTRUs, a BS, and an RNC.

FIG. 2 is a functional block diagram of a wireless transmit/receive unit(WTRU) and the BS of FIG. 1.

FIG. 3 shows a sample TDD frame structure.

FIG. 4 shows an example ranging RA region and a BS's ranging RAresponse.

FIG. 5 shows an example of a BS's ranging RA response per RA region orper RA types when a RA region is shared by multiple RA types.

FIG. 6 is a diagram of an example of a single BS RA response message forcontention-based bandwidth request.

FIG. 7 shows an example of an uplink data collision after RA requestswhen HARQ is not used.

FIG. 8 shows an example of an UL data region collision after RA requestwhen UL HARQ is used.

FIG. 9 is a diagram of an example using BS-Assistance information in aRA timer or a negative acknowledgement (NACK).

DETAILED DESCRIPTION

1. Introduction

When referred to hereafter, the terminology “wireless transmit/receiveunit (WTRU)” includes but is not limited to a user equipment (UE), amobile station (MS), a fixed or mobile subscriber station (SS or MS), anadvanced mobile station (AMS), a pager, a cellular telephone, a personaldigital assistant (PDA), a computer, or any other type of user devicecapable of operating in a wireless environment. When referred tohereafter, the terminology “base station (BS)” includes but is notlimited to a BS, a site controller, an access point (AP), an advancedbase station (ABS), a Node-B, or any other type of interfacing devicecapable of operating in a wireless environment. The solutions andmechanisms described herein may be applicable to TDD, FDD, and othersystems.

FIG. 1 shows a wireless communication system 100 including a pluralityof WTRUs 110, a BS 120, and a radio network controller (RNC) 130. Asshown in FIG. 1, the WTRUs 110 are in communication with the BS 120,which is in communication with the RNC 130. The WTRUs 110 are configuredto receive data transmissions from the BS 120 over a high speed shareddata channel. The BS 120 and/or WTRUs 110 are configured to detectrandom access (RA) failures and collisions, as described herein.Although only three WTRUs 110, one BS 120, and one RNC 130 are shown inFIG. 1, it should be noted that any combination of wireless and wireddevices may be included in the wireless communication system 100. Forexample, although the RNC 130 is shown in the wireless communicationsystem 100, the RNC 130 may not exist in the system 100 and may beincluded in the BS 120 or any other entity in the system 100. It shouldbe understood that the communication between the WTRUs 110, BSes 120,and RNC 130 or other networks like the Internet may be done usingpacket-based communication.

FIG. 2 is a functional block diagram 200 of a WTRU 110 and the BS 120 ofFIG. 1. As shown in FIG. 2, the WTRU 110 is in communication with the BS120 and both may be configured to detect random access (RA) failures andcollisions, as discussed below.

In addition to the components that may be found in a typical WTRU, theWTRU 110 includes a processor 115, a receiver 116, a transmitter 117,and an antenna 118. The processor 115 is configured to detect RAfailures and collisions. The receiver 116 and the transmitter 117 are incommunication with the processor 115. The antenna 118 is incommunication with both the receiver 116 and the transmitter 117 tofacilitate the transmission and reception of wireless data.

In addition to the components that may be found in a typical BS, the BS120 includes a processor 125, a receiver 126, a transmitter 127, and anantenna 128. The processor 125 is configured to detect RA failures andcollisions. The receiver 126 and the transmitter 127 are incommunication with the processor 125. The antenna 128 is incommunication with both the receiver 126 and the transmitter 127 tofacilitate the transmission and reception of wireless data.

2. Example Frame Structure

FIG. 3 shows an example of a TDD frame structure that currentlycorresponds to the IEEE 802.16m TDD frame structure, although otherframe structures can of course be used with the embodiments describedherein. Regardless of channel bandwidth (5, 7, 8.75, 10, or 20 MHz), thesuperframe 310 may be 20 ms in length as shown and may be divided intofour 5 ms frames 320. The frames 320 may be further divided into 5-8subframes 330. These subframes 330 may each contain five, six, seven, ornine OFDM symbols, depending on the type of subframe 330 in use.

Each TDD subframe 330 contains a downlink portion 340 and an uplinkportion 350, separated by a TTG (Transmit/Receive transition gap) 360and an RTG (Receive/Transmit transition gap) 380, inserted betweentransmit and receive, or receive and transmit, respectively.Downlink/Uplink ratios may vary for the 5, 7, 8.75, 10, and 20 MHzbandwidths. Some examples of Downlink/Uplink ratios are: 8/0, 6/2, 5/3,4/4, 3/5.

FIG. 3 shows the frame structure for 5, 10, and 20 MHz TDD with 5/3DL/UL split.

A single downlink portion 340 may contain multiple bursts of varyingsize and type carrying data for several WTRUs. The size could also bevariable on a frame-by-frame basis. Each burst may contain multipleconcatenated fixed-size or variable-size packets or fragments of packetsreceived from higher layers.

The uplink portion 350 may be made up of several uplink data bursts fromdifferent WTRUs. A portion of the uplink portion 350 may be set asidefor contention-based access, also called Random Access (RA), that may beused for a variety of purposes, including ranging for UL, time, andpower adjustments during network entry as well as periodicallyafterward. The RA channel may also be used by WTRUs to make uplinkbandwidth requests. In addition, best-effort data may be sent on thiscontention-based channel, particularly when the amount of data to sendis too small to justify requesting a dedicated channel.

3. Random Access (RA)

In scheduling based wireless broadband access systems, Random Access(RA) refers to an uplink (UL) region that is allocated for WTRUs toaccess. It is also referred to as contention-based access. The UL regionallocated for RA may be called a Random Access (RA) region.

A RA region commonly includes RA channels, where a RA channel isphysically modulated to carry certain information codes. The informationcode carried by a RA channel is called a RA code. In some RA codedesigns, multiple codes with certain orthogonal properties can betransmitted by different WTRUs in the same RA channel. For example, inIEEE 802.16 systems, a Code Division Multiple Access (CDMA)-like RA codemay be used, so that one RA channel may be accessed by multiple WTRUswith different RA codes.

A RA opportunity may refer to the opportunity that a WTRU uses totransmit its RA request. When a RA channel may carry multiple RA codes,a RA opportunity may be a combination of a RA channel and a RA code.When a WTRU needs to send a RA request, it may transmit a specific RAcode in a RA channel.

After sending a RA request, the WTRU waits for the BS to provide aresponse to its RA request that is identified by the RA opportunity thatthe WTRU used to send the RA request, e.g., the location of the RAregion, RA channel, and RA code. The expected response varies with theintention of the WTRU's RA request.

3.1 RA Use Cases

The RA opportunities for different RA use cases may be identified bydifferent RA regions allocated by the BS or by different RA codedomains/different RA channels in the same RA region. At least four usecases of Random Access (RA) may exist: initial access for a newsubscriber (WTRU) to join the network (initial ranging); Handover of asubscriber (WTRU) from one BS to another BS (HO ranging); periodicmaintenance of the uplink (UL) transmission parameters (periodicranging); and bandwidth requests for existing subscribers (WTRUs)(contention-based bandwidth request).

Initial ranging is a process for a new WTRU to initiate communicationwith a BS. The WTRU sends an initial ranging request in an initialranging RA opportunity to the BS, for example, an initial ranging codein an Initial Ranging RA channel. After sending its initial rangingrequest, the WTRU waits for a response from the BS. The expectedresponse to an initial ranging request may be a ranging statusnotification with or without UL transmission parameter adjustments.

HO ranging is a process for a WTRU to initiate communication with thetarget BS during handover from one BS to another BS. The WTRU sends a HOranging request in a HO ranging RA opportunity to the BS, i.e., a HOranging code in a HO ranging RA channel. After sending its HO Rangingrequest, the WTRU waits for a response from the BS. The expectedresponse to a HO ranging request may be a ranging status notificationwith or without UL transmission parameter adjustments.

With respect to periodic ranging, since each WTRU may be a differentdistance from a BS, it may be important in the uplink to synchronizesymbols in both the time domain and frequency domain and equalize thereceived power levels among the various active WTRUs. In the uplink, theactive WTRUs may need to be synchronized to at least within a cyclicprefix guard time of one another. Otherwise, significant intercarrierand intersymbol interference may result. Similarly, although downlinkpower control may be used in order to reduce spurious other-cellinterference, it may not be strictly required. Uplink power control may(1) improve battery life, (2) reduce spurious other-cell interference,and (3) avoid drowning out faraway WTRUs in the same cell that aresharing an Orthogonal Frequency Division Multiplexing (OFDM) symbol withthem.

This periodic ranging process, when initiated, may require the BS toestimate the channel strength and the time/frequency of the received ULsignals from the WTRU in question and then send the necessary ULtransmission parameter adjustments, e.g., time, frequency, and/or powerlevel, to the WTRU. When the WTRU uses unicast UL allocation, the BS mayestimate the received unicast UL data from the WTRU to determine if ULtransmission parameter adjustment is needed. When the WTRU does not haveunicast UL allocation and the periodic ranging is initiated, the WTRUneeds to use a periodic ranging RA opportunity to send the BS a periodicranging request. After sending its periodic ranging request, the WTRUwaits for a response from the BS. The expected response to the periodicranging request may be a ranging status notification with or without ULtransmission parameter adjustments.

A contention-based bandwidth request may be a process that a WTRU usesto request UL bandwidth by sending a Bandwidth Request (BR) RA request.The WTRU may send a BR RA request in a BR RA opportunity to the BS,i.e., a BR RA code in a BR RA channel. After sending a BR RA request,the WTRU waits for a response from the BS. The expected response to a BRRA request may be an UL bandwidth grant.

4. Embodiments Addressing Latentcy and Collisions

4.1 Latency

Within this or other systems, there are several embodiments that mayaddress the latency issue of RA failure detection at the WTRU.Generally, the embodiments may use BS assistance to help the WTRUs inthe RA-failure detection so that the WTRUs may timely and accuratelydetect the failures of their RA attempts and then start their RArecovery processes accordingly.

In a first embodiment, the BS may send responses to all the received RArequests in a RA region in the same aggregate BS RA response message,and this aggregate BS RA response may be sent at the earliest possibleBS RA response opportunity. Depending on the purpose of the RA request,the BS RA responses may be ranging status notifications with or withoutparameter adjustments, resource allocations, or simple acknowledgementsindicating a successful reception of the RA request.

In the event that no RA request is successfully received by the BS in aRA region, the BS may not transmit the response message or the BS maytransmit a null message.

Upon receiving and decoding a BS RA response message that containsresponses to the successfully received WTRU RA requests, a WTRU maydeterministically detect if its own RA request was successfully receivedat the BS. The WTRU does this by checking whether the aggregate BS RAresponse message contains a response to its RA request, for example, aRA channel plus a RA code in IEEE 802.16 systems. If the aggregate BS RAresponse message does not contain any response to its RA request, theWTRU may consider that its RA request failed and then start its RArecovery process. This recovery process could begin immediately, withoutwaiting for a time out, such that the latency of RA failure detection atWTRU side is significantly reduced.

Another benefit of using an aggregate BS RA message may be overheadreduction as the BS sends the responses to all the received RA requestsin the same aggregate BS RA response message, compared to sending anindividual TA response message to each RA request. The overheadreduction comes from saving multiple message headers and having a moreefficient way to identify the RA opportunities in one aggregate messagerather than identifying them each individually in separate messages.

4.1.1 Initial Ranging Request

For example, the BS may allocate a ranging RA region in the UL in orderto provide initial ranging opportunities for the new WTRUs to join thenetwork. A new WTRU, after acquiring the essential system configurationparameters and synchronizing with the downlink (DL) of the BS, may startthe initial ranging process by sending an initial ranging request, i.e.,transmitting a selected initial ranging RA code in a selected initialranging channel. After receiving and decoding an initial ranging region,the BS may send an aggregate RA response message at the earliestpossible BS initial ranging RA response opportunity. The aggregate RAresponse message may contain initial ranging RA responses to all thesuccessfully received and decoded initial ranging RA requests in theinitial ranging RA region. For each successfully received and decodedinitial ranging request, the BS's RA response may contain the rangingstatus and the necessary UL transmission parameter adjustments.

As shown in FIG. 4, after an UL initial ranging region 1010, theearliest BS initial ranging RA response opportunity may be determined bythe BS's scheduler that manages and allocates the radio link resourcesbased on the BS's real-time traffic load and traffic composition. For aninitial ranging region in frame 1000, the earliest BS initial ranging RAresponse opportunity 1035 may be a DL portion 1020 of frame 1030, i.e.,the next DL portion after the UL portion with the initial ranging region1010, if the BS scheduler can accommodate the initial ranging RAaggregate response message in it after other higher priority trafficdata. Otherwise, the earliest BS initial ranging RA response opportunitymay be the RA response opportunity 1045 in the DL portion of frame 1040or later. This may be due to heavy DL traffic with higher priorities.

After sending an initial ranging RA request, the WTRU waits for theinitial ranging RA response from the BS. Once the initial ranging RAresponse message is received and decoded, the WTRU attempts to identifythe response to its request based on the used initial ranging RA channeland initial ranging RA code. If the BS aggregate initial ranging RAresponse message contains a response to the WTRU's request, the WTRU mayconsider its initial ranging RA request successful and move to the nextstep according the given response. If the BS initial ranging RAaggregate response message does not contain a response to the WTRU'srequest, the WTRU may consider its initial ranging RA request failed,and it can immediately start the initial ranging RA recovery process.

The use of a BS initial ranging RA response message containing theresponses to all successfully received and decoded initial ranging RArequests in an initial ranging region may make it possible for the WTRUsto timely and deterministically detect the status of their initialranging RA attempts. If a WTRU detects a failure of its initial rangingRA request, the initial ranging RA recovery process may be startedimmediately, minimizing the latency in the initial ranging RA failuredetection and recovery.

Comparing this to a method where the BS sends separate initial rangingresponses to each successfully received and decoded initial ranging RArequest, the aggregation of the responses to the successfully receivedand decoded initial ranging RA requests in one BS initial ranging RAresponse message reduces the overhead of MAC encodings, as it savesmultiple MAC headers, and, also, it may have a more efficient way toidentify the RA requests when aggregating them together.

An initial ranging RA timer may still be used to handle the exceptions,such as message delivery errors, in order to improve overall systemrobustness.

4.1.2 HO Ranging Request

A similar embodiment may also be applied to the HO ranging RA processwhere the HO ranging RA opportunities may be in a separate RA region.Alternatively, as shown in FIG. 5 the HO ranging RA opportunities mayshare the same RA region 1110 as initial ranging opportunities, as shownin frame 1100. In other words, in FIG. 5, the shared RA region 1110contains ranging RA opportunities for initial ranging and HO ranging.For example, in IEEE 802.16 systems, the initial ranging and HO rangingmay share the same RA region where they are distinguished by usingdifferent RA codes.

When the HO ranging shares the same RA region with the initial ranging,the BS may send one RA response message 1135 containing the responses toall successfully received and decoded initial ranging RA requests and HOranging RA requests at the earliest BS initial/HO ranging responseopportunity (in frame 1130 for the HO ranging RA request). In FIG. 5,the BS ranging RA response message 1135 contains responses to allreceived RA requests, including initial ranging RA requests and HOranging RA requests in the previous ranging RA region. Alternatively,the BS may send two separate aggregate RA response messages 1138, 1145,one for initial ranging and one for HO ranging, at the earliest BS HOranging RA response opportunity and the earliest BS initial ranging RAranging response opportunity in frames 1130 and 1140, respectively. InFIG. 5, the BS sends two separate RA response messages, one containingresponses to all received initial ranging RA requests and the othercontaining responses to all received HO ranging RA requests, with thesame or different response time. The earliest BS initial ranging RAresponse opportunity may be different from the earliest BS HO ranging RAresponse opportunity, depending on the BS scheduler's decision regardingthe priorities of those RA responses and the BS's real-time traffic loadand traffic composition.

4.1.3 Periodic Ranging Request

In addition, a similar embodiment may also be applied to a periodicranging RA process. Similarly, the periodic ranging RA opportunities maybe in a separate RA region. In this case, after each periodic ranging RAregion, the BS sends a periodic ranging RA response message containingthe RA responses to all successfully received and decoded periodicranging RA requests at the earliest possible BS periodic rangingresponse time.

The periodic ranging RA region may have a different PHY channel designfrom the initial ranging/HO ranging RA region, as it may be for therandom access from the WTRUs that have been synchronized with the BS inthe UL.

Alternatively, the periodic ranging RA opportunities may share the sameRA region with other ranging types, e.g., initial ranging and/or HOranging. In this case, after each ranging RA region, the BS may send onesingle ranging RA response message containing the RA responses to allsuccessfully received and decoded ranging RA requests including allranging types at the earliest possible BS ranging RA response time.Alternatively, the BS may send multiple ranging RA response messages,one for each ranging type or any combination of the ranging typessupported in the ranging RA region, at the same or different earliest RAresponse time.

4.1.4 Contention-Based Request

In another embodiment, with the contention-based bandwidth Request RA,the BS RA response messages may contain the resource allocations thatmay be located in the next UL region or spread in the next multiple ULregions. As shown in a simplified frame shown in FIG. 6, for the RAregion 602 in frame 600, the BS sends a BS RA aggregate response message611 in the DL 612 of frame 610, which contains the responses to all thecorrectly received RA requests, where the responses may contain theresource allocations 616 in frame n 610 or the next several frames 620.In FIG. 6, the aggregate response message 611 is one single BS RAmessage containing responses to all received RA requests. Actual ULallocations may spread into next multiple UL regions.

The BS RA aggregate response message 611 may also indicate a futuresub-frame where a UL resource allocation control signal, e.g.,Advanced-MAP (A-MAP) in IEEE 802.16m, containing the actual allocationmay be found.

In this way, the BS RA aggregate response message 611 also serves as theacknowledgement to the WTRUs for their bandwidth request RA requestssent in the previous bandwidth request RA region. That is, the BS RAaggregate response message may serve two functions: one to allocateresources; and the other to acknowledge WTRUs that used the previous RAregion.

For a WTRU that used a RA opportunity for an UL bandwidth request in theprevious bandwidth request RA region, if the WTRU receives the expectedresponse from the BS RA aggregate response message, it may consider thatits RA attempt was successful and also know where its UL unicastallocation is. If the WTRU does not receive its expected response in theBS RA aggregate response message, it may recognize that its RA attemptfailed, and it may start the recovery process immediately or at suchtime as makes sense.

4.1.5 Additional Embodiments Regarding Responses

In another embodiment, after sending a RA request, if the WTRU does notreceive a response in the aggregate RA response message from the BS, itmay start the RA recovery procedure, unless a maximum number of allowedRA attempts have been reached.

Alternatively, in another embodiment, an explicit ACK may be sentimmediately upon detection of the RA request. For a RA request thatrequires resource allocation, the actual resource grant may be sent at alater time when resources are available.

Upon receiving an ACK for a RA request requiring resource allocation,the WTRU may start a timer. Expiration of the timer withoutgrant/response may be a cause for the WTRU to retry the accessprocedure. This may prevent the WTRU from waiting too long in the eventof a reception error.

In another embodiment, in the BS RA aggregate response message, thesuccessfully received and decoded RA requests are identified by the RAopportunity descriptors, e.g., the RA region, the RA channel, and the RAcode (as used, for example in IEEE 802.16 systems), or identifiersderived from the RA opportunity descriptors. If the RA opportunitydescriptors are used in the BS RA response message for a RA region, thecoding efficiency may be improved by using a two-level list, i.e.,listing the RA channels with at least one received and decoded RA codeand then, for each listed RA channel, listing all received and decodedRA codes, as compared to using a one-level list of the pairs (RAchannel, RA Code) for the received and decoded RA opportunities.

In another embodiment, the ACK may be sent per code and RA channelcombination, together called a RA opportunity. RA opportunities may bepartitioned into groups. For each group, all successfully received RAsare signaled. For each access group with N opportunities, up to Kmaxreceived RAs may be signaled. The number K may be signaled, whichrequires log₂(Kmax) bits. An index to the combination of successful RA'smay be signaled, which requires log₂(N/K) bits. An index may be reservedto signal the event where K>Kmax.

In another embodiment, the BS assists the WTRUs to have an improved timeperiod for the RA timer that is dynamic and varies with the systemloads. For example, for the RA resource allocation requests, when thesystem is lightly loaded, the required resources can be allocatedquickly, therefore the RA timer value can be smaller. When the system isheavily loaded, then the required resources will be allocated spreadingin the time domain, therefore the timer value should be larger. It isdifficult or impossible for the WTRUs to dynamically change their RAtimer because the WTRU does not have the system loading informationwithout being informed by the BS. With BS assistance, after the BSreceives/processes a RA region, it sends out a broadcast message,telling the WTRUs the RA timer values for the given RA region, based onthe BS's knowledge about how long it will take to finish responding tothose correctly received RA requests. Alternatively, sending time isunspecified, and the WTRU follows the last RA timer value available.

When multiple classes of RA requests are supported, if the RA classinformation is provided in the received RA requests, the BS may alsosend multiple RA time values, each for a specific RA request class. AnRA timer setting broadcast message may be very short, as it onlycontains small numbers, i.e., the RA timer values. This embodiment mayalso apply to the detections of both collision-caused RA failure andpower-too-low-caused RA failure. If the WTRU does not receive theexpected RA response within the given time, it may start the RA recoveryprocedure unless the maximum number of allowed RA attempts has beenreached.

4.2 Collisions

In a first method to address collisions, all wireless transmit/receiveunits (WTRUs) may use distinct codes; however, due to receiverlimitations, more codes are used than can be detected. The first resultmay be that not all codes are detected, but those that are detected arecorrect. This is a problem only if all WTRUs choose the same channelagain. The second result is that false detections are made. This wouldbe a low probability event, and the recovery from it would be in averification stage.

A first embodiment may address the UL data collision problem caused bymissed-detection of RA collisions when UL Hybrid Automatic RepeatRequest (HARQ) is not used. In FIG. 7, an RA region 1210 containsmultiple RA request opportunities requiring UL resource allocation(e.g., contention-based bandwidth requests). A missed-detection of RAcollision happens when the BS erroneously detects a single RA request ina RA opportunity that had a RA collision, i.e., accessed by two or moreWTRUs. If the expected RA response 1235 may involve a unicast UL dataallocation, for example, in the contention-based UL bandwidth request RAprocess, the BS may allocate a unicast UL allocation to the decoded RArequest. However, two or more WTRUs that used the same RA opportunitymay then transmit in the UL allocation 1238, thus resulting in dataregion collision.

When UL HARQ is not used for the UL transmission, the involved WTRUs maydetect the UL data region collision by a failure of receiving theexpected response from the BS to their UL data if their UL data requirescertain response, for example, as part of hand-shaking protocol data;otherwise, the involved WTRUs may not be able to detect such an UL dataregion collision at the MAC layer. The proposed embodiment may use theBS's assistance to the WTRUs in detecting such UL data regioncollisions. The BS has the knowledge about which UL data allocation isfor a RA response, and the BS also has the knowledge if the UL dataregion has been successfully received. Therefore, in case of UL dataregion collision, the BS can use its knowledge to send an indication tothe WTRUs to inform them of the failure of reception of the UL dataregion allocated for a RA request. Such an indication of UL data regioncollisions may be called an RA-initiated UL dataNegative-ACKnowledgement (NACK).

When the WTRUs that used the given UL allocation receive such aRA-initiated UL data NACK from the BS, they may realize that an erroroccurred in their “on-going” procedure, and an error recovery is needed.

In FIG. 7, the BS sends a NACK 1245 indicating a failure of reception ofthe UL burst allocated for the RA request. In a RA-initiated UL dataNACK 1245 sent by the BS, the UL data region may be identified by the RArequest to which the UL data allocation was given as the response, wherethe RA request may be identified by the RA request descriptors, forexample, the RA region, the RA channel, and the RA code, or anidentifier derived from these RA descriptors.

In a RA-initiated UL data NACK 1245 sent by the BS, the UL data regionmay also be identified by the UL data allocation descriptors, forexample, frame index, subframe index, LRU (Logical Resource Unit) index,etc., or an identifier derived from these UL allocation descriptors.

The BS may send a RA-initiated UL data NACK 1245 as a MAC controlsignal. For example, in IEEE 802.16 systems, such a RA-initiated UL dataNACK may be encoded as a control message, a control signaling header, ora subheader or extended header of a MAC PDU (Protocol Data Unit), or anAdvanced-MAP (A-MAP) Information Element (IE).

Another embodiment may address the UL data collision problem caused bymissed-detection of a RA collision when UL HARQ is used. In this case,the UL data collision caused by missed-detection of a RA collision maycause substantial resource waste, because it may force the collidedWTRUs to repeatedly retransmit and hence re-collide until reaching themaximum allowed number of HARQ retransmissions. With UL HARQ, detectingan UL data region collision may require additional attempts and/or othertechniques than just normal UL data HARQ decoding, because a singleburst error may be difficult to tell if it is the result of normal linkerror or a collision.

As shown in FIG. 8, multiple RA request opportunities are transmitted ina random access region 1310 of frame 1300 in the uplink, and, in theframe 1301, the BS aggregate RA response message 1335 provides responsesto all received RA requests in the random access region 1310, some ofwhich may provide UL resource allocations (e.g., contention-basedbandwidth requests), and other requests. After receiving and decodingthe first UL transmission in an UL data allocation as the response to aRA request in the random access region 1310, the BS may still be able todetect a collision. For example, the BS may consider that a collision isdetected if the UL data region 1338A, 1338B experiences an abnormallyhigh level of errors based on the channel coding, such as IncrementalRedundancy HARQ (IR HARQ), or if it observes other abnormal physicalsignal changes, e.g., channel delay spread. In FIG. 8, a UL resourceallocation in the UL data region 1338A, 1338B is made as a response toan RA request that was a missed-detection of a collision (i.e.,erroneously detected as a successful RA request but was actually acollision (i.e., sent by more than one WTRU)). Consequently, more thanone WTRU transmits in this UL allocation, thus resulting in a UL dataregion collision.

When receiving and decoding a subsequent UL HARQ retransmission butbefore reaching the maximum number of HARQ retransmissions, the BS maybe able to detect a collision in an UL data allocation as the responseto a RA request. For example, the BS may consider that a collision isdetected if it observes no gain has been obtained from the UL HARQretransmissions.

Once detected, an UL data region collision may be handled by using theBS's assistance to help the WTRUs to timely detect the error condition,stop UL HARQ retransmissions, and enter an error recovery process. TheBS assistance may be an indication signal sent by the BS to the WTRUs,e.g., a RA-initiated UL data Negative ACKnowledgement (NACK) 1350 in thedownlink in frame 1303 in FIG. 8. In FIG. 8, the NACK 1350 from the BSmay be, for example, a control signal terminating the synchronized ULHARQ allocations, indicating a failure of reception of the UL burstallocated for the RA request. UL resources in regions 1355A and 1355B inframes 1303 and 1304, respectively, of FIG. 8, originally allocated forsynchronized UL HARQ retransmissions, are now used for other UL traffic.

When receiving such a RA-initiated UL data NACK 1350 from the BS, theWTRU may stop the HARQ retransmission process and enter the randomaccess recovery process immediately.

Similar to the previous embodiment, the RA-initiated UL data NACK 1350signaled by the BS may be encoded as a MAC control signal, for example,in IEEE 802.16 systems, a control message, or a control signalingheader, or a subheader or extended header of a MAC PDU (Protocol DataUnit), or an Advanced-MAP (A-MAP) Information Element (IE).

In addition, if the synchronized UL HARQ is used, the RA-initiated ULdata NACK sent by the BS may be the same control signal used toterminate the synchronized UL resource allocations for UL HARQretransmissions, for example in frame 1303 in FIG. 8. For example, inIEEE 802.16 systems, a CDMA allocation A-MAP IE with zero allocation canbe used by the BS to signal the RA-initiated UL data NACK and also toterminate the synchronized UL resource allocations for UL HARQretransmissions.

Also, if the non-synchronized UL HARQ is used, the RA-initiated UL dataNACK signaled by the BS may be a combination of UL HARQ NACK and no ULresource allocated for the HARQ retransmission within a pre-defined timeinterval after the previous HARQ transmission or retransmission.

Alternatively, if the non-synchronized UL HARQ is used, the RA-initiatedUL data NACK signaled by the BS may be a combination of UL HARQ NACK andzero UL resource allocation for the HARQ retransmission. For example, inIEEE 802.16 systems, zero UL resource allocation may be a CDMAallocation A-MAP IE with zero allocation.

When using the zero allocation for the UL HARQ retransmission for an ULallocation following a RA request to terminate its UL HARQretransmission process, the intended WTRUs may be identified by thepreviously used RA opportunity descriptors, for example, the RA region,the RA channel, and the RA code, or an identifier derived from these RAdescriptors.

Alternatively, when using the zero allocation for the UL HARQretransmission for an UL allocation following a RA request to terminateits UL HARQ retransmission process, its intended WTRUs may be identifiedby the previously used UL data allocation descriptors, for example,frame index, subframe index, LRU (Logical Resource Unit) index, etc., oran identifier derived from these UL allocation descriptors.

When using the zero allocation for the UL HARQ retransmission for an ULallocation following a RA request to terminate its UL HARQretransmission process, the identifiers of its intended WTRUs may beincluded in an information field in the zero allocation control signalor may be masked with the Cyclic Redundancy Check (CRC) of the zeroallocation control signal.

The zero allocation control signal may be encoded as a stand-alone ULresource allocation information element, for example, a stand-aloneA-MAP IE, where its type value indicates the zero UL allocation.

Alternatively, the zero allocation control signal may be encoded as aspecial case of the UL allocation IE for the UL allocation following aRA request, for example, the CDMA Allocation IE, where the special casemay be indicated by a specific value of an information field in the ULallocation IE.

Another embodiment may address the UL data collision problem caused bymissed-detection of RA collisions when UL HARQ is used. The BS mayconfigure a limited number of UL HARQ retransmissions for the UL dataregion allocated as a response to a RA request, where such a limitednumber is less than a maximum number of UL HARQ retransmissions forother UL allocations. With this embodiment, the WTRU that transmits inan UL allocation following a RA request may exit the UL HARQretransmission when it has transmitted the transmission n timesincluding the original transmission, where1≤n≤Max_num_HARQ_retransmissions. The value of n may be decided bysystem configuration. The n-th HARQ NACK from the BS also indicates thetermination of the HARQ retransmission process. When receiving the n-thHARQ NACK, the WTRU terminates the UL HARQ retransmission and may enterthe random access recovery process.

Another embodiment may address the collision problem where not all codesare detected and where nothing is detected. This embodiment uses arecovery mechanism such that the WTRU may randomize its choice of slotand codes.

Another embodiment may address the RA collision problem where a RAcollision is detected. In this example, the BS may send a NACK to notifythe WTRUs that the RA failures that the BS detected in the previous RAregion are due to collisions. The benefit of sending a NACK and not anACK is in reduced overhead as NACK signals are the exception rather thanthe rule.

FIG. 9 is a diagram of a frame using BS-Assistance information in a RAtimer or a NACK. When a BS detects a collision in RA, at the next DLopportunity, the BS may send a NACK message 613 to inform thesubscribers of the detected collision. This will simultaneouslyeliminate/minimize the false detection at the WTRU side and reduce theRA latency significantly. The proposed NACK mechanism may be an additionto the WTRU timer-based mechanism, not a replacement. A timer-basedmechanism may be employed to be the backup mechanism in certainscenarios.

The embodiments discussed herein may address the detections of bothcollision-caused RA failure and insufficient signal power RA failure, aswell as other causes of RA failure.

Although features and elements are described above in particularcombinations, each feature or element can be used alone without theother features and elements or in various combinations with or withoutother features and elements. The methods or flow charts provided hereinmay be implemented in a computer program, software, or firmwareincorporated in a computer-readable storage medium for execution by ageneral purpose computer or a processor. Examples of computer-readablestorage mediums include a read only memory (ROM), a random access memory(RAM), a register, cache memory, semiconductor memory devices, magneticmedia such as internal hard disks and removable disks, magneto-opticalmedia, and optical media such as CD-ROM disks, and digital versatiledisks (DVDs).

Suitable processors include, by way of example, a general purposeprocessor, a special purpose processor, a conventional processor, adigital signal processor (DSP), a plurality of microprocessors, one ormore microprocessors in association with a DSP core, a controller, amicrocontroller, Application Specific Integrated Circuits (ASICs), FieldProgrammable Gate Arrays (FPGAs) circuits, any other type of integratedcircuit (IC), and/or a state machine.

A processor in association with software may be used to implement aradio frequency transceiver for use in a wireless transmit receive unit(WTRU), user equipment (UE), terminal, base station, radio networkcontroller (RNC), or any host computer. The WTRU may be used inconjunction with modules, implemented in hardware and/or software, suchas a camera, a video camera module, a videophone, a speakerphone, avibration device, a speaker, a microphone, a television transceiver, ahands free headset, a keyboard, a Bluetooth® module, a frequencymodulated (FM) radio unit, a liquid crystal display (LCD) display unit,an organic light-emitting diode (OLED) display unit, a digital musicplayer, a media player, a video game player module, an Internet browser,and/or any wireless local area network (WLAN) or Ultra Wide Band (UWB)module.

What is claimed is:
 1. A wireless transmit/receive unit (WTRU)comprising: a processor; and a transceiver, the processor and thetransceiver are configured to acquire a first random accessconfiguration and a second random access configuration, wherein thefirst random access configuration and the second random accessconfiguration are different, the processor and the transceiver arefurther configured to perform a first random access procedure using thefirst random access configuration using first physical resources and asecond random access procedure using the second random accessconfiguration using second physical resources, wherein the first randomaccess configuration is used for at least initial random access and thesecond random access configuration is used for timing adjustments,wherein the first physical resources are different physical resourcesthan the second physical resources, and the processor and thetransceiver are further configured to receive a first message for thefirst random access procedure and to receive a second message for thesecond random access procedure, wherein the first message is anaggregated response message including a plurality of first random accessresponses and the second message is an aggregated response messageincluding a plurality of second random access responses.
 2. The WTRU ofclaim 1, wherein the second random access configuration is used forperiodic random access.
 3. The WTRU of claim 1, wherein the processor isfurther configured for each of the first message and the second messageto identify one of the random access responses as being for the WTRUusing a code indicated in the identified random access response.
 4. TheWTRU of claim 1, wherein the first random access procedure is furtherused for handover random access.
 5. The WTRU of claim 1, wherein thesecond random access configuration is further used for poweradjustments.
 6. The WTRU of claim 1, wherein the first random accessconfiguration includes at least one random access code for use in thefirst random access procedure and the second random access configurationincludes at least one random access code for use in the second randomaccess procedure.
 7. The WTRU of claim 1, wherein the second randomaccess procedure is for WTRUs that have been synchronized with a basestation in the uplink.
 8. The WTRU of claim 7, wherein the first randomaccess procedure is for WTRUs that have been synchronized with the basestation in the downlink.
 9. A method comprising: acquiring, by awireless transmit/receive unit (WTRU), a first random accessconfiguration and a second random access configuration, wherein thefirst random access configuration and the second random accessconfiguration are different; performing, by the WTRU, a first randomaccess procedure using the first random access configuration using firstphysical resources and a second random access procedure using the secondrandom access configuration using second physical resources, wherein thefirst random access configuration is used for at least initial randomaccess and the second random access configuration is used for timingadjustments, wherein the first physical resources are different physicalresources than the second physical resources, and receiving a firstmessage for the first random access procedure and receiving a secondmessage for the second random access procedure, wherein the firstmessage is an aggregated response message including a plurality of firstrandom access responses and the second message is an aggregated responsemessage including a plurality of second random access responses.
 10. Themethod of claim 9, wherein the second random access configuration isused for periodic random access.
 11. The method of claim 9, furthercomprising identifying, for each of the first message and the secondmessage, one of the random access responses as being for the WTRU usinga code indicated in the identified random access response.
 12. Themethod of claim 9, wherein the first random access procedure is furtherused for handover random access.
 13. The method of claim 9, wherein thesecond random access configuration is further used for poweradjustments.
 14. The method of claim 9, wherein the first random accessconfiguration includes at least one random access code for use in thefirst random access procedure and the second random access configurationincludes at least one random access code for use in the second randomaccess procedure.
 15. The method of claim 9, wherein the second randomaccess procedure is for WTRUs that have been synchronized with a basestation in the uplink.
 16. The method of claim 15, wherein the firstrandom access procedure is for WTRUs that have been synchronized withthe base station in the downlink.